Asphalt concrete is a useful material in the road construction industry. Federal and state guidelines require that asphalt concrete laid at certain thicknesses must have certain properties that evidence its safety and long-term performance. If these guidelines are not met, the roadway surface will fail over time when exposed to severe conditions of heat, cold, and moisture. Therefore, samples of asphalt concrete roadway material must be tested to determine proper composition and properties.
When employing composite materials, for example, a bituminous paving mixture, it is generally desirable to test the composition of the materials before installation to ensure that the installed material has desired properties of structural strength, durability, etc. For example, the "hot-mix" asphalt concrete used to pave roads, airport runways, etc., desirably has a predetermined proportion of asphalt binder to aggregate, and a predetermined gradation of aggregate size to help ensure that the material will have adequate and uniform application and wear properties.
Pyrolysis techniques which provide for both content and gradation analyses are known whereby the asphalt binder in a sample of asphalt is burned off to leave an aggregate residue. Pyrolysis techniques are generally described in "Historical Development of Asphalt Content Determination by the Ignition Method," by Brown et al., and in "Solvent-Free, Nuclear-Free Determination of Asphalt Content and Gradation of Hot-Mix Asphalt Concrete," by Todres et al., ASTM Journal of Testing and Evaluation, November 1994, 564-570.
According to these techniques, a sample of asphalt concrete is heated to volatilize and combust the asphalt binder, thus separating the binder from the sample and leaving an aggregate residue. However, insufficient temperatures may not completely separate the binder. Excessive temperatures can lead to aggregate loss and gradation changes induced by chemical changes and thermal shock in the aggregate. Several furnace-type apparatuses have been developed for performing asphalt pyrolysis, including furnaces which incorporate an integral weighing scale in order to allow measurement of a sample of asphalt concrete during pyrolysis as described in, for example, U.S. Pat. No. 5,081,046 to Schneider et al.
Variations in characteristics at installation sites also may lead to variation in combustion conditions. For example, a specimen of hot-mix asphalt may be divided into several samples which may be processed in different furnaces, even different furnaces at different testing sites. Variable combustion conditions in any of the furnaces may lead to inaccurate or nonuniform results among the furnaces. Moreover, nonoptimal combustion may lead to deleterious side effects such as poor emissions quality, formation of soot deposits in the furnace and exhaust system, and gaseous discharges into the testing site which may be harmful to personnel and equipment. Afterburners and filters may trap or burn some pollutants which otherwise might be discharged, but still may not produce the combustion and exhaust characteristics to the levels needed to reduce unwanted emissions.
Under currently practiced protocols, hot asphalt concrete samples are placed in stainless steel trays and positioned within a furnace that is pre-heated to an elevated temperature, typically in excess of 500.degree. C. Inside the furnace, the sample is heated by conductive and convective heat transfer to achieve ignition. Heating the sample to the ignition temperature and thereafter combusting the asphalt binder content can require several hours or longer. Weight loss is measured during combustion by an internal balance incorporated in the furnace floor, and final asphalt content is determined.
It has now been discovered that these processes may be inherently inaccurate due to such factors as incomplete combustion, mineral loss, and aggregate gradation changes. For example, furnace temperatures may reach levels for periods of time that cause decomposition of some of the aggregate as well as the binder. In particular, extensive heating can cause cracking and decomposition of the aggregate, resulting in loss of aggregate from the sample and reduction of the aggregate particle size, adversely affecting the accuracy of the overall assay.